HelloController.php

<?php

/**
 * @file
 * Contains \Drupal\hello\Controller\HelloController.
 */

namespace Drupal\hello\Controller;

/**
 * Returns responses for hello module routes.
 */
class HelloController {

  /**
   * Show “Hello from Drupal 8” message.
   *
   * @return array
   *   The message.
   */
  public function hello() {
    return array(
      '#type' => 'markup',
      '#markup' => t('Hello from Drupal 8'),
    );
  }
}